Electromechanical filter



Aug. 4, 1953 W. VAN B. ROBERTS ETAL ELECTROMECHANICAL FILTER Filed March 50, 1949 2 Sheetls-Sheet l Aug. 4, 1953 w. VAN B. ROBERTS Erm. ELEc'rRoMEcHANl-CAL FILTER Filed March 50, 1949 2 sheets-sheet 2 W i n F -/0 /32 {24- 30 31 y 31 30 l25 .las "im 1^ N 5 5 52 27 26 29 26 29 2a 2'/ gg @3L/f Wim-w+? m rwl I .l

INVENTOR .walter van. er

ly ATTORYLEYq Patented ug. 4, A1951" ELECTROMECHANICAL FILTER Walter van B. Roberts and Leslie L. Burns, Jr.,

Princeton, N. J., assignors to Radio Corporation of America, a corporation of Delaware Application March 30, 1949, Serial No. 84,372

(Cl. S33-71) 12 Claims. 1

This invention relates to electromechanical iilters, and more particularly to band pass lters of the mechanically-vibrating type.

A vibrating metal resonator has a rather low decrement compared to an electrical circuit. For example, the Q of a mechanical resonator or tank is of the order of a few hundred for nickel, several thousand for various steels, and as high as ten thousand for aluminum and its alloys. Furthermore, at radio frequencies the metal tank is small and cheap compared to the` corresponding electrical tank, so that many of such metal tanks can be used in a lter. Thus, the high Q and small size of metal resonators make them desirable for use in multi-section filters for low radio frequencies.

An object of this invention is to devise a simple type of band pass filter composed of loosely coupled metal resonators.

Another object is to devise band pass filters which have extremely narrow pass bands.

A further object is toprovide various expedients by the use of one or more of which the pass bands of electromechanical filters may be narrowed.

A still further object is to devise a small, simple and cheap band pass lter for low radio frequencies.

The foregoing and other objects of the invention will be best understood from the following description of some exemplications thereof, reference being had to the accompanying drawings, wherein:

Fig. 1 is a schematic view of the basic filter unit of the invention;

Fig. 2 is a representation of another embodiment of a basic iilter unit;

Fig. 3 represents the transmission line analogy of Fig. l;

Figs. 4a, 4b, 4c and 4d are schematic representations of various types of single section iilters;

Fig. 5 is a section through a slug-type lter according to this invention;

Fig. 6 is a section through a modified necktype filter of this invention;

Fig. '7 is a section through another embodiment of a iilter of this invention;

Figs. 8 and 9 are schematic representations of further embodiments of the invention;

Fig. 10 is a partly diagrammatic representation of another lter according to the invention: and

Fig. 11 is a schematic representation of a multisection iilter having a. plurality of sections like those of Fig. 4c.

The basic unit of the iilters to be described is a single section lter composed of a pair of similar longitudinal resonators loosely coupled together so as to possess a pair of resonant frequencies marking the limits of its transmission band. Fig. 1 shows, as one way of making such a section, a pair of half-wave-length metallic resonators or tanks I and 2 connected together by a metallic neck coupling portion 3 so thin as to act like a weak spring. Both ends of each resonators I and 2 are motional loops at each of the resonant frequencies.

On the other hand, Fig. 2 showsa pair of quarter-wave metallic tank resonators 4 and 5 coupled by a metallic slug 6 which is relatively thick, so that it acts somewhat as a heavy mass. In this case, only the free ends of the resonators 4 and 5 are motional loops.

In the cases of both Figs. 1 and 2, the tanks I, 2 and 4, 5 are longitudinally vibratory elements connected by coupling elements 3 and 6 adapted to transmit vibrations from each tank to its neighbor.

The band limits in either case can be determined by calculating the kfrequencies at which the tanks are resonant when vibrating in the same phase and in opposite phases. However, to obtain a more complete picture it is desirable to make use of the fact that these filter sections are mathematically equivalent to electrical lters composed of equal lengths of transmission line connected by a piece of line of different characteristic impedance. This equivalence may be made use of by assuming that the tanks are composed of elements of uniform cross-section and the couplers also of uniform cross-section, but of different mechanical impedance. The term mechanical impedance is here defined as the product of the density of the material, its cross-sectional area and the velocity of compressional waves along the tank or coupler.. respective tanks and couplers of Figs. 1 and 2 are all made of the same material, the mechanical impedance of the tanks will be different from that of the couplers, due to their different crosssectional areas, the tanks having higher impedance than the coupler in Fig. 1 and vice versa in Fig. 2. Both Figs. 1 and 2, `however,.disclose single section filters, that is to say filters comprising two tanks and a single coupler.

Fig. 3 shows the electrical filter section corresponding to Fig. 1. Its tank portions have electrical length 01 and characteristic impedance Z1 while the connecting portion has length 02 and impedance Zz.

If the' The pass bandsfor a filter eom" I fact that the square of the iterative impedance is the product of the input impedanceswhenjV the section is open-circuited and shQrt-circuited By either method'lie sqiiarejvofw at its center. h, I lthe iterative impedance is fouiidlt'obe (tan 0pt-p tan (p cot @-itan 0,) 2: 2 2

Fmtan2 (i1-25mn 01 cot H02- in whichexpression p stands for the ratio LZ2/Z1..

vSince a pass band occurs'in any range of H1 in which the above lexpressions are positive, it

is evident that band limits are given by the roots ofbot'h the numerator and denominator of these expressions. If Ap is either very small orv veryr largey the roots occur in 'closely spaced,r pairs,

i. e., the pass bands are narrow.` Within a pass band denedby a pair of' roots of the numerator it is evident that the iterative impedance falls tozero at the band edges Aand has, `a maximum near mid-band. Denominator rootsLonQthe other hand, dene bands in which 'the iterative impedance rises to iniinity at the bandiedges and is minimum at mid-band.

In a given structure the ratio H2/0i is fixed as the frequency is varied, so that Expressions 1 and 2 can'be simpliiied for a number` orV particular values of this ratio., For, example`if' pairs of band edges isgiven 92:91, a series of These 4expressions hold good forY either neck-l type or slug-typel iilters theformer type beingj defined Vas one `having ka value of p small' corn-1 paredto 'unity' while the latter has p large corn-jV paredto' ,unity. v Perhaps'theV most important" casein practice to which the' above equations] apply is the' slug-type iilter with quarter-wave tanksand quarter-wave slug, shown in Fig. 4c

and'in Fig. 2, ',The rstof'the twov equationsA is used since a large value of tan 0i is required.`

In this case vthe angular difference betweeifi the." values 'of 01 satisfying the equation is readily'de-v termined to be approximately fory large values of p. Since the total angular,

length lof theV two tanks is 1r/2, the` fractional band widththat theratio of bandwidth. in

cycles per second, is

(1 1) 1f P P It may be noted in passing that the equationsV indicar@ the exis-@eig pass andermans 4 coupling elements are a multiple of a half-wave in length, but the fractional band width in this case is determined by the square root of p (or its reciprocal, according to the type of filter), so that in such case an impractically great disparity between tank and coupler impedances is required to obtain a narrow band.

Another important case invpracticeis for which a straightforward solution gives an aPDXQXma-Lei-fractional band width for tanks a half-wave long and small p, again with a.quar,ter -,wave coupling element. Again, there'rareother pass bands of lesser interest for the saine structure, which structure is shown in Fig. 4b and in Fig. 1.

4One more particular case of importance isY with large p. This covers a section composed of resonators three quarter-waves long coupled by a slug' one-quarter wave long,I shownin u 4a, This is a desirable se ':tior'1"'ff rl 'a' 'slugtype lter because the drive andtake-off resonate(rs1` have motion'al nodes. 'available' 'for the coupling' coils. n The fractional band ini this case is apl" pmxmatelyl 4 .t M

4 l 1, 1Y 31u11 '310) for large values of p.,

. Comperingfihi--bamlfwith the band previously.

shown in Fig. 4a as quarter-waver .reSQnatOrgu coupled by a quarter-wave neck., :l The fractional band width in this case is approximately,

4 ptlnp It may be noted, from the foregoing equations, thatpqr i" l must be proportional to or of .thecrdeiyof .the .Y

desired fractinal bandwidth,

While the band widthpanube expressed.,ex. plicitlyviorvarighs simplf'g'` relations between mi' and 02 as illustrated above, 't canalso be 'determined4 approximately' in the 4general'case,at least for the'casxe'f of narrow bands'vfFor, referring back to'nquationsi and 2, the'rootsof tha-numerator occur when tan Y If, now, the band is suiciently narrow so that sin 02 can be considered :as constant throughout the band, then the angular bands between roots are approximately l2p'3 Sill 62 for p very small, and

p sin 62 for p very large. From these expressions, it is evident that in every case the band width is minimum when sin 02=1, i. e., when the coupling element is an odd multiple of a quarterwave long. This is an important feature of the present invention. However, it will also be noted that considerable departure from the quarterwave optimum length of coupler element is possible without much increase in band width.

A reasonably convenient graphical solution for Equation 1 which applies to any ratio 62/01 can be carried out as follows: Two circles are drawn with their centers on the origin, one with radius unity and the other with radius p.

Straight lines are drawn tangent to each circle at its top, bottom, and two sides. A line from the origin at angle 01 intersects lines tangent to the unit circle at points proportional to tan 01 and cot 01. Another line at angle intersects the p circle tangent lines at points giving p tan and p cot By revolving the two lines from the origin while keeping the ratio of their angles constant, the value of 01 may be determined which makes the numerator of Expression 1 vanish. The same construction will give all the roots. However, as this method of analysis is not likely to be required in view of the solutions already obtained for the more important practical cases, it will not be elaborated in further detail. From Figs. 4a-4d, approximate band widths for tanks of :greater length can be inferred by assuming the band width varies inversely as the tank length, while the band width is substantially unaffected by the length of the coupling element, so long as it is an odd multiple of a quarter-wave. Since the band width is narrower the greater the tank length, the band may be narrowed by adding an integral number of half-wavelengths to the tank. In general, for a given p the band is narrowest when the coupler is a quarter-wave long and is also narrower the longer the tank.

It should be noted that Figs. 1 and 2 may represent either gures of rotation or structures cut out of sheet metal, or any other structure having uniform impedance along each of its distinct portions. A multisection filter may be built up by connecting several of the single-section units end-to-end, the pass band and impedance being the same as for the single sec tion. By way of example, Fig. 11 shows a twosection slug-type lter made of two sections as shown in Fig. 4c connected end-to-end.

Practical limitations on narrowness of band width ,In a mechanical lter, the characteristic or mechanical impedance of each portion is the product of its cross-sectional area and the intrinsic impedance of the material of which it is made, the latter quantity being in turn the product of its density and the velocity of propagation of longitudinal waves along that portion. If all of the lter is m-ade of the same material, the quantity p is simply the ratio of cross-sections, assuming that the sections are small enough so that the sound velocity is the same in all portions. As has been demonstrated, p or l/p must be of the order of the desired fractional band width. If a 1 per cent band is required, this means that a neck-type filter turned out of round stock must have a neck diameter only about one-tenth that of the tanks. It is easy to see that narrowness of band is limited by iiimsiness of the structure. Matters are even worse if the iilter is cut out of nat strip material (which would otherwise be a desirable construction because of the ease of punching such filters out in quantity), because in this case the neck width must be p times the tank width.

At this point, it might be thought that while there is a limit to the smallness of p practically obtainable in neck-type lters, p could be made as large as desired in slug-type lters by simply making the slug large enough. Unfortunately, however, if the diameter of the slug is made too large it begins to vibrate in various undesired or spurious modes. Such undesired responses could perhaps be dodged by careful design or could even be employed to provide rejection points outside the pass band, but for present purposes it appears preferable to keep the design as noncritical as possible by keeping all extraneous responses well away from the desired pass band.

A simple expedient for narrowing the band of the type of section heretofore discussed, without changing the practically obtainable value of p, is to use longer tanks. This, however, cannot be carried too far, as it results in an inconveniently long filter if many sections are employed, and also brings other pass bands too close to the desired band.

In accordance with an aspect of the present invention, another expedient for narrowing the band width which may be employed, in slugtype filters, is to use for the slugs (or for the higher impedance elements), at least in part, a material having higher intrinsic impedance than the material used for the tanks or low impedance elements.

As an example of this expedient, Fig. 5 shows a nlter section composed of a cylinder l of high impedance material, such as steel or nickel, which is drilled and soldered to a cylinder 8 of low impedance material, the cylinder 1 thereby forming a slug on tank B. The tank cylinder or rod 8 may be aluminum, which has a low intrinsic impedance, with nickel plating. This nickel plating not only makes it possible to solder the aluminum tank 8 to the steel coupling element or slug 1, but also, in accordance with the principles disclosed in the copending Burns application, Serial No. 84,373, filed March 30, 1949, now Patent No. 2,619,604, issued November 25, 1952,

afname 7i makesE possible magnetostrictionl' driving f', and take o'fff'roin, theta'nks;

Another' expedient which constitutesr a further phase of' the? present inventiorr is to use'thinwalled nickel tubing for the tank =8 of Fig. 5, so as to" reduce its cross-sectional area without a corresponding loss of stur'diness. In other words, by using tubingfor the tank, the ratio p is greatly increased without making the structure unduly flimsy; Also, the eddy current losses are lessened, compared with solidrod,when inserted in driving and take-offL coils: l

Both-of the expedients just discussed- (that is, the useof materials of differentpintrinsic impedance for tanks and coupling elements, andthe use of tubing rather than'wrod) are also applicable to neck-typelters. Fig. 6 is'an'A illustration of this concept.- In said gure, the two high :impedance tanks- 9 and I0 of nickel or steel are drilled and soldered onto thetwo end portions of :a thin-walledv tube H of low impedance material such as aluminum', elements 9 and` I0 thusforming tanks which are coupled by a neck-type coupling element il. Alternatively, the coupling element Il could' be a rody rather thanl a tube if desired.

By the use of one or more of the abovedescribed expedients, pass bands suiciently narrow for many purposes have been obtained without undue flimsiness of structure.

Fig. 'l illustrates a further' modioa-tion that has proved very satisfactory in practice. 'I'he structure of Fig. '7 is somewhat simila-r to that of Fig. 5, but in Fig. 7 a steel ball bearing I2 (annealed for easy drilling) is soldered on a nickel' ro'd I23`. Although elernent i3 is' shown as a rod, it is desired? to be' made clear that thinwalle'd nickel tubing may be utilized for element f3', rathei than a rod, and in fact it has been found preferable to' dc so. In Fig. 7, the steel ball i2 is substituted' for the' cylindrical coupler l ofi Fig'. 5. l

While filter of Figi. 7 is not readily analyzed rnathenia'tioally,v it has vbeen found that a ball diameter' of about a quarter-wavelength is su-itable, the band width being controlled by the choice of tube' -o'r rod diameter and tube wall thickness Experience indicates that balls are less likely' to' develop extraneous resonances than the corresponding cylindrical slugs because the lowest natural Yfrequency of a ball a quarterwavel'er'igftl in diameter is nearly twice the operating frequency. Thus, if the tank can be made to give the' desired bar-ld width, the ball is less susceptible to' spurious' modes' of vibration than the corresponding cylindrical coupler of Fig. 5. The lowest natural frequency o'f a'l steel ball one inch in dianre'ter'is about 100 kilocycles, while for other sizes the frequency is inversely proportional to diameter'.

The structure of Fig. 7 may be termed a ballcoupled section',v and a plurality of such sections may be placed end-to-end to form a multiplesection filter' somewhat similar to the multipleneck and multiple-slug types to be described later in connection withmigs. 8 and 9. Of course, the placing of the Fig. '7 sections end-to-end has been describedmerely for illustrative purposes; actually, such a multiple-section ball-coupled filter would be constructed by mounting a plurality of b'alls' at appropriate intervals on a single section ofv tubing or rod. Such a multiple-section ball- `ccui'ple'd filter can be made to have a narrow .enough'pass-band for most purposes.

tried, with more or less success.

Filterafor oem/marrow bands- From the foregoing, it may be seen that to obtain bands as'narrow astliehigh Q of aluxninum makes possible, or to obtain moderatelyl narrow bands in the case of filters punched out of strip or sheet stock, some radically different method for obtaining sufciently 1oose= coupling is necessary. Many such methods have been For example, the coupling element mayv be connected between points on the tanks near motional nodes, where the same coupling element i's less effective. Again, linear tanks may be coupled by inertia effects, for example, b'yf one or more small steel ballbearings pressed between the sides of parallel tanks; the different' kinetic energieslmparted to the .balls when the tanks vibrate in and. out of .phase producetwo natural frequencies" in the' system and the'band' width' isl'ess the nearer the balls are located' to motional nodes of the tanks.

But the simplest and most satisfactory method so far discovered Afor obtaining a very narrow band is to use what will be called'multiple-neck or multiple-slug coupling elements. This arrangement permits extremely loose coupling between tanks without requiring excessive ratios of impedances of the various parts of the filter.

Figs. 8 and 9 represent neck and slug-type sections coupled by twinelements.

In Fig. v8, two high-impedance tanks I4 and 1.5 are coupled by twin quarter-wave necks IE and I'l separated by a quarter-wave slug IRT. Fig. 8 therefore shows high impedancev tanks coupled by multiple necks.

In Fig. 9, two low-impedance tanks I9 and 20 are coupled by twin quarter-wave slugs 2l and 22 separated by a quarter-wave neck 23. Fig. 9 therefore shows low-impedance tanks coupled by multiple slugs.

A physical picture of the operation of both of these coupling systems, for example that of Fig. 8, may be had by considering one element, say i6, adjacent to a tank I4 as the actual coupling element while the other two, say I8 and IT, act as two quarter-wave transformers in tandem which transform the impedance of the other tank l5 to a value still further out of line with that of the coupling. element i6, the impedanceV transformation, more specifically, being. equal toA the square of th'e ratio of their individual impedances. This results in a narrower band than that givenv by Figs. l andZ for the same ratio of impedances of the elements. The central elements I3 and 23 ofthe sectionsV of Figs. 8 and 9 need not be of the same impedance as th'e tanks, but it facilitates analysis and is a convenient construction.

The pass bands andv vterminating impedances of Figs.- 8 and v9 may be determinedy exactly as in the case of the simple coupling elements, by obtaining' an expression for the iterative impedance of the section. The derivationV is, of course, more lengthy and leads to a more complicated expression for the square of the iterative impedan'ce, namely,

make Expression 3 positive.4 The bands of chief interest are those which occur when the elements 9 are in the vicinity of a quarter-wave long, or an odd multiple thereof. The band width may be accurately computed by means of Expression 3, but for practical purposes it is usually suncient to figure that the band is narrower by the factor i? (or i for slug-type filters) as compared to the corresponding filter with a single coupling element.

In Fig. 9, the two slug portions 2| and 22 are separated by a fully-defined neck portion 23, while in Fig. 2 there is a single slug 6 separating the two low-impedance tanks 4 and 5. rIt has been found, according to this invention, that it is possible to achieve some of the desirable or narrowebandwidth results of the twin-type filter of Fig. 9 by modification of the Fig. 2 embodiment without, however, going to a fully-dened neck portion intermediate two slugs as in Fig. 9. In other words, it is possible to obtain some of the advantages of the Fig. 9 design while utilizing essentially only the single-slug construction of Fig. 2. More particularly, if the slug 6 of Fig. 2 is 3A-wavelength long, a Vshaped notch may be cut out of the central portion of slug 6, for example by the use of a lathe, this slot extending entirely around the circumference of slug 6. The band width obtainable by this expedient varies with the depth of the notch, the deeper the notch, the narrower the band width, band here referring to the pass band of the lter. Such a notched-slug construction provides a wider pass band than the twin-element construction of Fig. 9 and a narrower pass band than the single-unit slug-type filter construction of Fig. 2.

Although in Fig. 8 the slug I8 has approximately the same transverse dimension as the tanks I4 and I'5, this does not necessarily need to be the case, as slug I8 may have either a larger or smaller transverse dimension than do the tanks I4 and I5, as long as the mechanical impedance of said slug has the proper relation to that of necks I6 and I1.

Another factor p may be obtained by adding another pair of quarter-wave elements to form a triple-neck or triple-slug coupling. Inr other Words, the band may be narrowed still further by adding more pairs of large and small quarterwave elements. Thus, by using a sufficient number of elements in the coupler it is possible to obtain as narrow a band as desired, without requiring p to be impractically large or small.

Fig. 10 shows, for example, a two-section iilter made of sheet nickel that has, in the design illustrated, one pass band from 94.7 to 97 kilocycles and another from 276 to 279.5 kilocycles. In this particular design, the large end tanks 24 and 25 were each one inch long, the end tanks each being joined to a large central element 26 by a series of three small quarter-wave elements 21, 28 and 29 and two large quarter-wave elements 30 and 3I, the large and small quarter-wave elements alternating between each end tank and the central element. The central element was two inches long, elements y2I--3I each f being 1A-inch long. The ratio of widths of the large and small elements, which in this case is p, was 3:1. The overall length of this filter, excluding permanent magnets, is nine inches. c

End tanks 24 and 25 are magnetized by permanent magnets 32 and 33 and are located in drive and take-off coils 34 and 35, respectively. The Fig. 10 arrangement is a two-section filter, in that each half of the structure, that is, one

end tank, half the central element, and the quarter-wave elements between them, is a single-section filter.

Another way of looking at the multiple coupler is to consider it as a low pass lter operating above cut off, and hence atttenuating vibrations passing through in either direction. From this point of view, it is evident that other forms of filters, operating in an attenuating band, could be employed as loose coupling means between tanks of the composite filter.

Filters including multiple-section coupling elements, and more particularly multiple-neck lters of the type illustrated in Figs. 8 and l0, are more particularly disclosed and claimed in the copending Roberts divisional application, Serial No. 172,746, filed July 8, 1950.

Distinction between sheet and turned filters As previously stated, it should be appreciated that a filter turned out of round stock will give a much narrower band, for the same ratio of coupling element diameter to tank diameter, than will one punched out of sheet material, because in a figure of revolution the impedance ratio p is the square of the dimension ratio. To give an illustration, it has been found easy enough to obtain a band of the order of cycles, at 100 kilocycles, in a filter like that of Fig. 9, by using nickel-plated 35" aluminum rod for the tanks, with two coupling slugs 2l and 22 per section comprising 1%, cylindrical steel slugs soldered on the aluminum rod.

The lters of this invention operate at frequencies up to a limit (at least 500 kc.) set chiey by the decreasing dimensions of the parts.

Choice of materials For the internal sections of any lter, it is generally desirable to use a material of the highest possible Q. For this reason aluminum would be the unquestioned choice, except for its large temperature coeflicient of frequency (about 200 parts per million per degree 0.). Where temperature stability is of primary importance, some isoelastic material, such as the nickel alloy Ni-Span C, may be used. This alloy has good magnetostrictive activity and better Q than nickel, but is not as yet obtainable in thin-walled tubing. It is available in standard wire gauges, and lters have been made by soldering steel balls on the wire. Although the Q of nickel is low compared to some other metals, it is still high enough for lters that are not extremely narrow band and do not require extremely sharp cut-off. These three materials are the ones that have been mostly used. The steels have high Q but have much higher intrinsic impedance than aluminum which, as will be shown later, makes it more difficult to terminate the lter nonreflectively. Thin-walled steel tubing, however, might be a good choice in some cases. Brass has a good enough Q and is easily nickel plated, but has relatively low compressional wave Velocity, so that the elements may be inconveniently short at high frequencies. The choice of material depends somewhat on the band width, the narrower the band, the lower loss the material should be to give sharp cut ofi.

yDrive and take-off methods For most purposes, the mechanical Jfilter must be driven from an electrical source and deliver power to an electrical load. The material used for the drive and take-off tanks must of course be magnetostrictive, if this type of electrome 11 chanical conversion is to,be used. Furthermore the mechanical losses in thesetanks neednot be so low as in the interior tanks. Sincelosses must be somehow embodied in oneorboth Aend tanks to terminate the 4iilter satisfactorily, Ait may be necessary to choose. a lossy,material in case the damping introducedbythetuned .coils on the ends of the filter is Inot suiiicient. For

many cases, thin nickel sheet or ,thin-walled nickel tubing makes a good endtank. AvFor. extremely narrow bands, however, nickel-plated aluminum comes nearer .to providingtherela tively low damping calledfor bythe narrowband.

In broad band lters, when the highest conversion efficiency is required, othermaterials may be utilized. The `main objection to .magnetostrictive operation is poor eficiencespecially at the higher frequencies, due to eddy current losses in materials such as nickel-or-.nickelalloys.

This drawback can, however, belargelyovercome .k

by employing a material with very vloweddy our rent losses. Ferrites, which are.,magnetc.ce ramic materials, vhave negligible.,eddyipurrent losses and a mechanical Q ofthe order.of,.one or two thousand. Also, ferriteshave beendeveloped which have as large a magnetostriction Vcoeiiicient as nickel, sc that considerable,damping` can be obtained by electrical reactions. `lftherend half-wavelengthelements of a filter be .replaced by half-wavelength elements .of ferrite..of the same characteristic impedance,a Y.very efficient conversion is possiblein partbecauseof the ,low mechanical and electrical..;losses.

Drive and take-oir ferrite ,resonators,f-.maywbe cemented to the lter metal, or .the ,ferritegela ments may be copper-platedatone,end fto-per mit soldering to the lter.

As an alternative .to a lmechanical joint, the ferrite element can be .held against the .remaining partof the filter by springpressureexerted l through an auxiliary low pass flter'-, Suchas previously describedin connectionvwith multipleelement couplers. VSuch a constructiongis more particularly described andclaimed-inythe copending .Roberts application,-gserialog, led May 14, 1949, now Patent v.No. 2,578,452, issued December 1l, 1951.

Fig. illustrates a drive and take-offlarrangement typical of all .theYmagnetostrictivelyeoper.- ated iilters of this invention. Closely-tting coils 34 and 35 are placed over the :middle of ,the end half-wave elements-24 and 25 .ofzthei iil'ter, to provide magnetostrietive v.coupling Permanent magnets 32 and 33vare.located so.as .to magnetize the .parts of -the lter Sunder.thecoilsinfa longitudinal direction. The .magneticfield,isad justed to give maximum .dr-ive, whichrequires that the material be something likehalfs atu rated; the shorter the pieceof -magnetostrictiye material, the stronger the eld required. The impedance of .the drivingcoil 1hasalarge resrstive component, due partly Ato losses the material and partly to the motional reaction .ofthe lter.

Teminations A nlter should be terminated in a resistance equal to the iterative impedanceof theA section, and this is not constant over the band, ashas been noted. Without some vform .of Adamping in the lter, the output consists ofv a` series of peaks with deep valleys between. If thelter is made of relatively low Q material such as ,riickelfand is suiciently narrow band, the peak-.to--valleyratio may be satisfactory without any -additional terminatingr resistance. Thesarne isivof v,course true of any material, if the band be mad-,Qagiow enough. But 4perhaps the Vonly way A,to get. la .perfect termination, Ywithout depending Aon losses withinthe lter, :is to extend the filter a fewrsections beyond 4the take-oir point and introduce .sufcientlosses-intolthese extra sections so that vibrations entering these sections are substantially damped out by the time they are rellected back to the pick-up point. This arrangement, however, allows most ofthe power transmitted to yiowppast.r the `pick-up point, rand yrelatively little `intorthe useful load.

vThe best compromise method so far discovered is .to g providethe tightest Vpossible magnetostricytive coupling between'the drive and take-off lter elements'and'their associated coils, which assures maximum -power transfer and also provides a corresponding amount of mechanical damping of -the end resonators, and then if the peak-to-valley lratio is still too great, to add mechanical resistanceauntilthe best-shaped response curve is obtained. 'It may be necessary to add mechanicalresistance or damping, in addition to the electromagnetic damping of the circuits due to the tightcoupling, where the band is wide or where the end tank losses are too low. It has been found that .a thin coating of silicone grease or vaseline,l applied to the end resonators, will provide :considerable damping. The viscosity of the silicone grease is less variable with temperature than is that of ordinary oils and greases.

A theoretically vbetter Way kto add a pure mechanical' resistance is to connect the end of the lter to a long rod of lossy material having a characteristic impedance chosen (by giving it the correct diameter) -to match the lter impedance somewhere inside the band. In practice, a rather short rod-shaped piece of an elastometric or Arubber-like` plastic vmaterial having a high damping factor, Lsuch as cellulose nitrate for ex ample, .cemented by 'its own solvent to the lter tip, `has beentfound to reduce very bad peak-tovalley ratios Atoan acceptable value.

v-Of course, Sit is desirable to provide as great as possible a proportionlof the required damping by the reaction of the drive and take-off circuits; for lthis reason, themechanical impedanceof the tanks Vshouldr be-made as low as possible in proportion to the magnetostrictive coupling. For example, if the take-01T tank is a nickel-plated aluminum tube of givendiameter and thickness of plating, then, 'the thinner the wall, the less 'the lterimpedance and hence 'the less the terminating resistance required. Thus, since the reaction dampingis constantfit should be possible to make the wall thin enough so that no extra damping isneeded. 'On the other hand, if theend tank is nickel-plated solid rod, it Will be seen that small diameter favors the ratio of magnetostrictive coupling to filter impedance.

Tuning-up methods The individual resonators of a lter should be tuned to the same frequency. It is probable that commercially-manufactured filters can be madel frequency o Ythe tank being tested, by the change of the apparent coil impedance at the resonant frequency of suck tank. It should be brought out that, for the purpose of testing the tanks of a complete lter, all the tanks not being tested must be so damped or detuned as to leave the frequency of the free tank in the test coil substantially unaffected by resonance of the others. This can usually be accomplished by putting tight-fitting clamps on the adjacent tank or tanks; gripping the adjacent tanks firmly between the fingers will sometimes be sufficient.

If it is found that any one resonator is too far out of line with the others, it is tuned either up or down by filing. Filing away material at motional loops increases the resonant frequency, while filing at nodes, to reduce its cross-section, decreases it. For decreasing the frequency of a neck-type filter, it is more of a reversible operation to put a ring or spot of solder on a motional loop at one or both ends of the resonator to lower the frequency, excess solder being filed off until the frequency is correct.

Torsion filters Any of the foregoing filters that have circular cross-section throughout will operate with torsional waves, the wave length, however, being about 60% of that of linear vibration at the same4 frequency, but being independent of the diameter of the element, like the linear vibration wave length, at least for sufficiently small diameters. This makes low frequency filters more compact, but is undesirable for frequencies so high that tanks are already inconveniently short.

Torsional operation has one important advantage, however, that may outweigh any disadvantages thereof, especially at frequencies so low that the drive and take-off angular vibrations may be converted to linear motions by simple mechanics. This advantage is that the quantity corresponding to characteristic impedance is in the case of torsional operation determined by the moment of inertia of the element about the axis and not by the cross-sectional area, and is proportional to the fourth power of the diameter rather than to the square thereof, as was the case for linear or longitudinal vibrators. So, a moderate ratio of diameters suffices to give a large ratio of impedances.

Thus, the same filter which gives a wide band when operated linearly will give a narrow band when operated in torsion, even though only a simple coupling element is used. Or, to put it another way, a narrow band torsion lter can` be made without a very great disparity in diameter between the tank and coupler portions.

In order to use a lter in the torsional mode, some modification of the drive and take-off arrangement is necessary. The simplest scheme that has been found is to nickel-plate only onehalf of the circumference of the drive and takeoff tanks and to apply a transverse magnetic field in the plane including the edges of the plating. The combination of the constant transverse magnetization with the alternating longitudinal magnetization produced by the driving coil gives a resultant magnetization which swings back and forth in direction about a mean position approximately transverse to the axis of the resonator, thus tending to twist the resonator ends first one way and then the other about the central plane.

Other methods for providing torsion drive include maintaining a steady bias torque on the resonator through some form of low-pass connection, or by obtaining bias twist of nickel plating 214 by -keeping the resonator twisted during plating and freeing it afterwards, or by twisting beyond the elastic limit before -plating and annealing afterwards. Or, the plating may be applied in thin spiral strips. In each case the coil 'and eld arrangements are the same as for longitudinal drive.

A particular application of the torsion filter is for extremely narrow band operation, which can be achieved by the use of multiple-neck coupling and which is made possible by the fact that magnesium has a uniquely high Q when operated in torsion, namely about 100,000.

`Torsional filters of the type describedabove are more particularly disclosed and claimed in the copending divisional application, Serial No. 166,518,1led June 7, 1950.

It will be seen, from all of the foregoing, that the objects of this invention have been accomplished. Band pass lters having extremely narrow pass bands have been devised. Moreover. small, simple and cheap band pass filters for low radio frequencies have been provided according to the teachings of this invention.

What I claim to be my invention is as follows:

1. A mechanical filter section, comprising a `pair of spaced resonant elements joined by a coupling element, each of said elements being adapted to transmit mechanical vibrations, said coupling element having a length of substantially an odd multiple of a quater-wavelength therein at the frequency of operation of said filter section and having a mechanical impedance different from that of said resonant elements and said resonant elements each having a length of substantially an odd multiple of a quarter-wavelength therein at said frequency.

2. A mechanical filter section, comprising a pair of spaced resonant elements each having a length of substantially an odd multi-ple of a quarter-wavelength therein at the frequency of operation of said filter section, and coupling means therebetween including a vibratile member having such dimensions that its resonant frequency falls outside the operating pass band of said section, said coupling means having a dimension equal to an odd multiple of a quaterwavelength therein and having a mechanical impedance different from that of said resonant elements.

3. A mechanical filter section as defined in claim 2, wherein said resonant elements are constituted at least in part by metallic rod.

4. A mechanical filter section as defined in claim 2, wherein the vibratile coupling member is a sphere.

5. A mechanical lter section as defined in claim 2, wherein the vibratile coupling member is a sphere the lowest resonant frequency of which is substantially above the operating pass band of the section.

6. A mechanical filter, comprising a plurality of spaced substantially similar resonators linearly arranged and having predetermined mechanical impedances, and at least one coupling element positioned between said resonators and adapted to couple compressional waves from one resonator to another, said coupling element having a mechanical impedance higher than the mechanical impedances of said resonators and having a maximum dimension of substantially an odd number of quarter-wavelengths therein at the frequency of operation of said filter, at least a portion of said element being made of a mate-- rial different from that of said resonators.

7.- Aa mechanical' lter,-'comprising a plurality 'of-spaced substantially similar resonators linearly arranged,- andlat least one coupling element positionedabetweens'aid resonators and adapted to couple -compressional Waves from one resonator to another,- saidcoupling relement having a crosssectional larea greater than that of said said resonators-.andhaving a maximum dimension of substantially an -foddnumber of quater-wavelengths therein atfthe frequency ofy operation of said-'fiilten atl least a-iportion of said element being-made of amaterial different from that of said resonators.

8.= A mechanicallter, comprising a plurality of spacedsubstantally similar resonators linearly arranged, and at least one coupling element positioned between and joining `adjacent ends of said resonators to each other, said coupling element having a cross-'sectional area greater than that of-:sai'd-resonators and having a maximum dimension of substantially an odd number of quarter-wavelengthsk at `the frequency of operation of: saidltensaid coupling element and vsaid resonators being made-of materials having dierent intrinsicv impedances, the intrinsic impedance of the-fcouplingfelement being greaterthan that of 'the resonators.

.5.9, Agmech'anicalxlter as defined in claim 6 whereinfsaidzcoupling element is a spherical metalliczmember.

10.-A mechanical :filterI as defined in claim 6 wherein said'resonatorsare in the form of metallic ztubing.

illfiAnmechanical-i'lter section comprising a pair of :spacedsubstantially similar resonators linearly arranged andzhaving predetermined mechanical impedances,- ysaid resonators each having.alengthof substantially an-o'dd multiple of L16 apunten-wavelength therein at`the frequency pf operation ofsaid filter section, and a coupling element'4 positioned-'between 'said resonators and .adapted'lto' couple 'cornpressional Waves from one :resonator `to' the other, said Acoupling element hav- 'iing 'a larger 'crossfsection'than said resonators and -a mechanical impedance higher than the mechanicalimped'ances of said resonators and having. a 'length "of substantially an odd number of quarter-wavelengths vtherein at' the frequency of operation of vsaid filter section.

12.Y A"mechanicallter comprising a plurality of mechanical llter sections as defined in claim -11 connectedfend-'to-'end WALTER vAN B.. ROBERTS.

:LESLIE L.rBURNs, JR.

References Cited in the le of this patent A-UNI'IEDSTATES PATENTS 1 OTHERREFERENCES Pierce, SMagnetostric'tion oscillators, Proceedings of the American'Academy of Arts and Sciences, vol. LXIII, No. 1, April 1928, pp. 39 and 40. (Copy in Scientific Library.)

Magazine, lillectronica vol. 20, No. 4, page 100, vApril' 1947. 

